Building mixins should combine duplicate nested selectors

I often find myself avoiding mixins because I know that the compiled CSS will have duplicate selectors, which makes for a larger file. For example, a reduced test case:

.mixin () {
    a {
        color: red;
    }
}
div {
    .mixin();
    a {
        text-decoration: none;
    }
}

When this is compiled, the resulting output is:

div a {
    color: red;
}
div a {
    text-decoration: none;
}

In a perfect world, selectors inside a mixin and selectors inside the scope in which the mixin was executed would be combined when they are identical. Then, the resulting output would be the much cleaner:

div a {
    color: red;
    text-decoration: none;
}

Start by compiling the reduced mixin test case in the issue and compare its duplicate CSS selectors with the requested combined output. Done means identical nested selectors from the mixin and its surrounding scope produce one CSS rule containing both declarations.